Lightwave networks may be used to rapidly transfer information among remote sites via optical signals that are transmitted through waveguides, such as optical fibers. In lightwave communications, data may be sent as an encoded optical signal with a known wavelength. The bandwidth of lightwave data that is transmitted through an optical fiber as a single optical signal is limited. Consequently, wavelength division multiplexing is performed to increase the transfer capacity of the optical fiber. Wavelength division multiplexing is the simultaneous transmission of multiple channels along an optical waveguide, with the channels being signals having different wavelengths. Typical wavelength division multiplexed optical signals include four, eight, sixteen or thirty-two channels. Each channel is also referred to as an optical carrier.
While wavelength division multiplexing allows the simultaneous transmission of multiple channels along a single optical fiber, it requires the multiplexed optical signal to be separated by wavelength at the receiving site, so that the individual channels can be distributed as needed. The separation of the multiplexed optical signal by wavelength is referred to as demultiplexing, which is often performed by Bragg gratings along one or more waveguides at the receiving site. As is well known, Bragg gratings are elements that establish periodic changes in refractive index, and the periodicity may be selected to correspond to a target wavelength in order to reflect a channel comprised of an optical signal having that wavelength.
One known configuration of a device for demultiplexing a wavelength division multiplexed optical signal is described in U.S. Pat. No. 5,457,760 to Mizrahi. An embodiment of the Mizrahi device is illustrated in FIG. 1. A demultiplexer 10 includes a coupling member 12 having an input fiber 14 and having eight output fibers 16, 18, 20, 22, 24, 26, 28 and 30. Each output waveguide includes seven of eight different wavelength-selective optical filtering elements 32, 34, 36, 38, 40, 42, 44 and 46. Typically, the filtering elements are fixed Bragg gratings. The eight different Bragg gratings are distinguishable with respect to the target wavelength. In FIG. 1, the eight different types of Bragg gratings are organized by target wavelengths into eight columns. Each one of the eight output waveguides 16-30 has a missing type of Bragg grating. For example, output waveguide 30 has Bragg gratings 34-46, but is missing Bragg grating 32. Thus, the output waveguide 30 will pass a channel having a wavelength that corresponds to the target wavelength of Bragg grating-type 32, but will reject the seven channels having wavelengths that correspond to the target wavelengths of Bragg grating-types 34-46.
In operation, the input fiber 14 of the demultiplexer 10 of FIG. 1 is connected to a source of a wavelength division multiplexed optical signal having eight channels, i.e., optical carriers. The wavelengths of the eight channels correspond to the target wavelengths of the eight types of fixed Bragg gratings 32-46. The eight-channel wavelength division multiplexed optical signal from the input fiber 14 is split into eight lower energy signals that are introduced into the eight output waveguides 16-30. The configuration of Bragg gratings along a particular output waveguide determines which of the eight channels will reach the end of the output waveguide opposite to the coupling member 12. Stated in the negative, the optical channel that is not reflected during propagation through a particular output waveguide is determined by which one of the eight alternative fixed Bragg gratings is missing from that output waveguide. As a result of the operation of the demultiplexer 10, the eight channels are isolated from each other at the ends of the output waveguide.
Isolating channels using devices such as the demultiplexer of Mizrahi allows a user to then manipulate the subsequent distribution of the channels by employing other optical processing equipment. For example, if a user desires to broadcast the two channels that are passed by output waveguides 16 and 18 of FIG. 1, downstream processing equipment may multiplex the two channels into a single wavelength division multiplexed optical signal, which is then transmitted to the various sites of the desired broadcast. The steps of demultiplexing the eight-channel optical signal and multiplexing two of the demultiplexed channels operate well for the intended purpose. However, each operation on an optical carrier imposes some attenuation and potentially introduces noise into the signal. Amplifiers may be used to reduce or eliminate the effects of the attenuation, but the amplification operation is another potential source of signal noise.
What is needed is an optical selector, such as a demultiplexer or sorter, which permits dynamic selections among different combinations of channels to be separated from a wavelength division multiplexed optical signal.